Human skin is very complex in terms of tonal shifts and colors. Depending on the light used during the shoot, you can achieve different results. Light decides everything: mood, composition, colors, tonal shifts, and so on. Then, during photo retouching, you will need to deal with things like color casts, skin blotchiness, imperfections of skin texture, shadow and highlight bending and uneven transitions between them, discoloration in highlights and excessive saturation in shadows, difference of skin color temperature or even hue depending on the size of the light source used, etc.
Photo retouching is an extremely powerful tool which allows me to complete my artistic vision. I do not care about how things should look like, all I care about is what feel and mood I want to convey. Photo retouching should not be a result of happy accident or what you think others will find appropriate, but a creative tool for finalizing YOUR idea. The same as with photography rules, you master those to break them. Composing a photo is like falling in love, you feel it, you do not think about it or analyze it. There are no limits to art, as long as it does not feel forced or fake. Photography and photo editing should be naturally inspired by what you see and what it makes you feel like. This is also why taking time to choose photo for retouching is of no less importance than the editing process. Not rushing through the retouch or using photoshop presets mindlessly will only benefit you at the end.
If the toning does not match the mood of the photo it will be a visual cacophony. You are composing a symphony, and all the instruments have to be in tune with one another. But understanding all this is not enough. You need a good connection with your model or client, a connection strong enough for you to either capture an emotion that is real and not staged, a moment in between if you will, or be able to evoke it. Only then you will fully understand the mood of your own photo. At that stage a direction in which you should take your retouching will become clear.
Private photoshop workshops via skype - http://www.ryuurui.com/photo-retouching-lessons.html
Photography workshops in Tokyo: http://www.ryuurui.com/photography-workshops.html
Hire a photographer in Tokyo: http://www.ryuurui.com/hire-a-photographer-in-tokyo.html
Photo blog: http://www.japan-in-photography.com/
For more tutorials and how to videos check out my photoshop and photography tips and tricks YouTube channel: https://www.youtube.com/channel/UCEOVGZ2rpLhR7gSPvaexxxQ